Dynamic Backcalculation Research Articles

Dynamic Back-Calculation Approach of Deflections Obtained from the Rolling Dynamic Deflectometer: Application and Validation

Dynamic Back-Calculation Approach of Deflections Obtained from the Rolling Dynamic Deflectometer: Application and Validation

Influence of bedrock on the dynamic deflection response and dynamic back-calculation results of asphalt pavement: Insights from the numerical simulation of falling weight deflectometer tests

The dynamic deflection response obtained by the falling weight deflectometer (FWD) test and back-calculated modulus results play an important role in road engineering. The presence of bedrock may affect the results of FWD tests on asphalt pavement. Many numerical simulation models of FWD tests have been established to obtain the dynamic response of pavement structures without considering the influence of bedrock, which prevents us from further understanding the influence of bedrock on dynamic response of asphalt pavements. Here, we consider the bedrock under asphalt pavement structures with an analysis program based on the spectral method with fixed-end boundary conditions (B-SEM). The B-SEM is employed to observe the dynamic response of a conventional asphalt pavement during the FWD test; the results are compared with those obtained by the finite element method (FEM). Semi-rigid base asphalt pavement, rigid base asphalt pavement, and flexible base asphalt pavement are simulated by the B-SEM with different bedrock depths, and the deflection results are compared and analyzed. Additionally, the bedrock’s influence on the dynamic modulus back-calculation results is investigated. The results show that compared with the FEM, the B-SEM produces results with considerable accuracy and high computational efficiency. Furthermore, shallow bedrock has an obvious influence on the dynamic deflection responses of different pavements, as well as the accuracy of the back-calculation results. The impact of bedrock on the modulus back-calculation of pavement layers is concentrated in the subbase and subgrade, with a minimal effect on the surface layer.

Influence of Bedrock on the Dynamic Deflection Response and Dynamic Back-Calculation Results of Asphalt Pavement: Insights from the Numerical Simulation of Falling Weight Deflectometer Tests

Influence of Bedrock on the Dynamic Deflection Response and Dynamic Back-Calculation Results of Asphalt Pavement: Insights from the Numerical Simulation of Falling Weight Deflectometer Tests

Dynamic Backcalculation Approach of Deflections Obtained from the Rolling Dynamic Deflectometer: Fundamental Theory

AbstractIn this study, the dynamic behavior of asphalt pavement from the Rolling Dynamic Deflectometer (RDD) was investigated. The effect of the loading frequency to the RDD deflections was evaluat...

Effectiveness of static and dynamic backcalculation approaches for asphalt pavement

In this study, the field falling weight deflectometer (FWD) data for asphalt pavement with various base types were backcalculated through dynamic and static backcalculation approaches, and the effectiveness of backcalculation approaches was studied. Asphalt concrete (AC) was treated as a viscoelastic material and the complex modulus was obtained using the dynamic approach. The dynamic modulus at a fixed frequency was computed for comparison purposes. The coefficient of variance and the compensating layer effect were assumed as two characteristics for the effectiveness of backcalculation approaches. The results show that the layer property from the dynamic backcalculation approach for different stations were more consistent and showed smaller coefficient of variance, which were more appropriate for the characterization pavement behavior. The elastic moduli from the static approach were more variable and exhibited a compensating layer effect in which a portion of the modulus of one layer was backcalculated into other layers. The dynamic approach is more effective than static approaches in backcalculation of layer properties.

Asphalt concrete master curve using dynamic backcalculation

ABSTRACT The dynamic backcalculation of pavement layer properties, and more specifically the ability to determine the asphalt concrete master curve, using Falling Weight Deflectometer (FWD) data has caught the attention of many researchers for years. The damaged or aged master curve of existing pavement structures allows the accurate prediction of the pavement structural capacity and remaining life – a goal that cannot be achieved through laboratory testing. To address this need, the authors developed a dynamic backcalculation application that employs the ABAQUS software for forward calculation using implicit dynamic analyses, and the Newton-Raphson’s root-solving algorithm for optimisation. The capabilities of the application were demonstrated for three simulated flexible pavement structures and two asphalt concrete mixes using 40 combinations. The asphalt concrete moduli at the FWD most dominant frequencies around 17 Hz, and the variables of unbound layers converged to the exact values in few iterations for all combinations. The master curves, and mainly the maximum moduli, were reliably determined to less than 1% and 4% error for about 60% and 85% of the combinations, respectively. The potential for determining the master curve and maximum modulus was shown to be dependent on the asphalt concrete mix, asphalt concrete temperature and pavement structure.

Determination of effective frequency range excited by falling weight deflectometer loading history for asphalt pavement

Falling weight deflectometer (FWD) tests have been widely used to evaluate the layer properties of in-situ asphalt pavement structures. The FWD loading is essentially impulse loading and can excite a wide range of frequency components. This study is to determine the effective frequency range excited by FWD loading through the dynamic back-calculation analysis of FWD data. The FWD deflections were calculated using the spectral element method for asphalt pavements under various field measured FWD loading histories. The calculated deflections were then used as the measured ones to back-calculate the properties of asphalt concrete layers using the dynamic back-calculation procedure. The viscoelastic behaviour of asphalt concrete was characterized using the modified Havriliak-Negami model. For each set of FWD data, several sets of back-calculated model coefficients were obtained by inputting different seed values of model coefficients in the back-calculation procedure. The results show that although all the back-calculated and measured deflection curves are in good agreement, the values of the back-calculated model coefficients can be significantly different and the back-calculated master curves match well only in a specific frequency range that is the effective frequency range excited by FWD loading. The effective frequency ranges for various pavement structures and FWD loading histories were determined. The results show that a higher loading peak level can excite a wider effective frequency range, or more specifically the greater upper limit, regardless of the pavement structures. In general, the effective frequency range excited by a FWD loading history is 5 ~ 65 Hz for asphalt pavement. The existence of effective frequency range indicates that the information contained in FWD data is not sufficient to capture the asphalt concrete properties over the entire frequency domain.

Comparisons of asphalt pavement responses computed using layer properties backcalculated from dynamic and static approaches

The falling weight deflectometer (FWD) testing is widely used as a nondestructive technique to backcalculate layer properties of in-situ asphalt pavement structures. Traditionally, static backcalculation methods based on the layered elastic theory (LET) are employed to analyze FWD data. However, the static method can not take into consideration the dynamic effects of the impulse loading generated in FWD testing, as well as the viscoelastic behaviour of asphalt concrete (AC). Therefore, the layer properties backcalculated from the static approach and the pavement responses computed using these layer properties are questionable. In this study, FWD tests were conducted on two asphalt pavements, one with a flexible base and the other with a semi-rigid base. The field FWD data were backcalculated using the traditional static approach and the dynamic approach which uses the spectral element method as the forward analysis engine. The elastic modulus of each layer was obtained from the static backcalculation, while in the dynamic backcalculation, the complex modulus master curve, which is the fundamental property of viscoelastic materials, was obtained for each AC layer, and elastic moduli were obtained for other layers. The two sets of backcalculated results were employed to analyze pavement responses to moving vehicular loading using the LET and layered viscoelastic theory, respectively. The results show that responses from the viscoelastic analyses are significantly larger than those from the elastic analyses except for the vertical strain at the asphalt treated base layer at high speeds. At a low speed of 16 km/h, most of the responses from the viscoelastic analyses are more than two times those from the elastic analyses. The results presented in this study pose a serious concern over the traditional approach in which both the backcalculation of FWD data and computation of pavement response to traffic loading are based on the LET.

A Comparison of Dynamic Soil Models for Backcalculation of Engineering Soil Properties from Falling Weight Deflectometer Data

The falling weight deflectometer (FWD) test is one of the most widely used nondestructive tests for backcalculation evaluation of pavement and soil properties. Despite the advantages in accounting for the time-dependent load and responses, the accuracy of evaluating elastic moduli based on dynamic backcalculation analyses is, however, significantly depended on the forward dynamic soil models employed in the backcalculation process. To serve this challenge, a comparison for advantages, limitations and the reliability of different dynamic soil models employed in the backcalculation of engineering soil properties is presented in this paper. Two types of dynamic soil forward models in the FWD backcalculation, namely, dynamic single-degree-offreedom (SDOF) and dynamic half-space models are considered. The study in this paper provides a better understanding of dynamic backcalculation processes which is essential for the development of dynamic backcalculation program and its applications.

Dynamic backcalculation with different load-time histories

This paper focused attention to the falling weight deflectometer (FWD) load-time history. For a commonly used device, it studied the pulse generation mechanism and the influence of different load histories on backcalculation results. In this connection, a semi-analytic impact theory was first introduced for realistically simulating FWD pulse generation. Then a newly developed finite-element code was presented for FWD interpretation; the code is capable of addressing dynamics, time-dependent layer properties, and quasi-nonlinear behaviour. Both new developments were demonstrated for an experimental dataset that resulted from operating an FWD with different loading configurations. It was found that backcalculated parameters are sensitive to the FWD pulse features. Consequently, it is recommended that, whenever advanced pavement characterisation is sought, experimental attention should be placed on generating diverse FWD pulse histories. Collectively, the resulting deflection histories will contain pertinent constitutive information for supporting the calibration of more complex pavement models.

Dynamic Soil Models for Backcalculation of Material Properties from Falling Weight Deflectometer Deflection Data

Falling Weight Deflectometer (FWD) test is one of the most widely used methods for in-situ nondestructive evaluation of pavement/soil properties and examination of the structural condition of in-service pavements. A dynamic half-space model is employed in the present work for backcalculation of engineering soil properties from the FWD data. The advantages, limitations and the reliability of the backcalculation elastic moduli evaluated from dynamic soil model are discussed in this paper. Selected numerical results are presented to portray the influence of governing parameters, for example, the presence of shallow stiff layer and the mass density of soil material on dynamic backcalculation of the soil elastic modulus. The investigation presented in this study provides a better understanding of dynamic backcalculation processes which is essential for the development of dynamic backcalculation program and its applications.

Dynamic Backcalculation of Viscoelastic Asphalt Properties and Master Curve Construction

In this study, dynamic backcalculation was conducted on two selected Long-Term Pavement Performance (LTPP) sections, one from a cold climate zone and the other from a warmer climate zone. It was demonstrated that not only the viscoelastic dynamic modulus (| E*|) of the asphalt has been backcalculated from an extensive set of field data but also the master curve has been constructed successfully with the backcalculated data. It was also found that the backcalculated master curves were significantly different from those constructed with the LTPP | E*| coefficients. Furthermore, the backcalculated dynamic modulus was obtained from the same pavement sections after 12 to 14 years of service and compared with the initial baseline modulus.

Subgrade Dynamic Backcalculation Based on PFWD

In order to evaluate the bearing capacity and the compaction performance of subgrade in a reasonable way, according to the time history results of pressure and displacement tested with the PFWD (Portable Falling Weight Deflectometer), the extraction methods of time history data were put forward to use for the backcalculation. Combined with the analytic solutions of dynamic response of vertical displacement on the half-space surface under the impact loading of semi-sine wave in the circular rigid vertical distribution for the dynamic half-space theory, the mathematical models of subgrade dynamic backcalculation were established based on the least square criterion, then the corresponding backcalculation flow and method were proposed in accordance with the modified Gauss-Newton algorithm. Firstly the pressure time history was fitted as a semi-sine wave to obtain the two backcalculation parameters of impact load, i.e. the angular velocity and the nonresponsive period. Secondly the displacement time history was fitted to obtain the two subgrade dynamic parameters, i.e. the dynamic modulus and the dynamic density. Lastly, through comparing the dynamic backcalculation to the static backcalculation from the tested data, the results show that the dynamic modulus is substantially equal to the static modulus, and there are the well correlations between the dynamic density and the dry density and the moisture content measured with the sand replacement method. Therefore, the PFWD can simultaneously test the design index (modulus) and the construction index (compactness) for the subgrade to achieve the both unified evaluation.

Static and Dynamic Backcalculation Analyses of an Inverted Pavement Structure

A crucial part of any maintenance strategy is an intricate understanding of the material characteristics of the pavement, so that the current level of damage may be accurately assessed and an appropriate plan implemented. Advances in the precision to which these parameters can be determined, as well as improvements in how these results are interpreted under varying conditions of measurement and analysis, are essential in the effective execution of a maintenance strategy. Results from Falling Weight Deflectometer (FWD), which is a Non-Destructive Testing (NDT) device, can be used to predict elastic modulus of any layer by comparing measured deflection data to calculated values through an iterative process referred to as back-calculation. This paper presents a comparison between static and dynamic back-calculation procedures, specifically with regard to typical South African inverted pavements. The analysis indicates a dynamic analysis provides results of greater accuracy than a static analysis, although the effect of the difference requires further investigation.

Viscowave – a new solution for viscoelastic wave propagation of layered structures subjected to an impact load

The forward models frequently adopted for use in the dynamic backcalculation of the falling weight deflectometer (FWD) data are based on the solutions that utilise discrete transforms. In this paper, a new computational algorithm, namely ViscoWave, has been developed and implemented for modelling the pavement dynamics under the FWD impact load. The primary advantage of the proposed solution over some of the existing solutions is that it uses continuous integral transforms (Laplace and Hankel transforms) that are more appropriate for the FWD time histories whose signal characteristics are transient, non-periodic and truncated. Sample runs of the ViscoWave and the validation efforts presented in this paper showed that the proposed algorithm is capable of modelling the dynamics of a layered structure with elastic or viscoelastic material with or without a halfspace, indicating the potential of ViscoWave as a forward model for dynamic backcalculation of flexible pavement properties.

An improved dynamic model for the study of a flexible pavement

This paper introduces the semi-analytical and finite element models implemented to study a Falling Weight Deflectometer test conducted on a flexible pavement. These dynamic models take into account the effects of both Rayleigh damping in soil and viscous damping in bituminous materials, with respect to temperature, on structural deflection. Moreover, numerical results have been compared with in situ measurements recorded on an instrumented pavement. Results from numerical models showed the importance of taking into account the effect of damping (hysteretic or viscoelastic) of all layers of the pavement against temperature, loading and mechanical parameters. The parametric analysis introduced as a basis for future development of a dynamic backcalculation program.

New dynamic procedure for back-calculating the shear strength parameters of large landslides

Case histories of the Tsaoling landslide and the Chiu-fen-erh-shan landslide, both located in Taiwan, are used in this study to validate the procedure proposed for the dynamic back-calculation of the shear strength parameters of the sliding surfaces of large landslides. This new procedure uses the Newmark sliding model while simultaneously considering the critical displacement concept. The premise of the new procedure is that, in a stable slope, back-calculation tends to underestimate the shear strength; however, as the intensity of earthquake shaking increases and the slope approaches failure, the back-calculated shear strength will approach to the “true” value. Based on this concept, the new procedure (method) is developed and validated with the two well-documented case histories. The shear strength parameters back-calculated using the proposed method agree well with those reported in the literature. The new dynamic back-calculation procedure is especially suitable in the situation where groundwater monitoring data are unavailable. Thus, the new method is complementary to the traditional groundwater-based limiting equilibrium back calculation approach.

Comparison of Static and Dynamic Backcalculation of Flexible Pavement Layers Moduli, Using Four Software Programs

Comparison of Static and Dynamic Backcalculation of Flexible Pavement Layers Moduli, Using Four Software Programs

STUDY ON STRUCTURAL EVALUATION OF PAVEMENT LAYER MODULI OBTAINED BY BACKCALCULATING FWD DEFLECTION DATA

Back calculation by which pavement layer moduli can be estimated from surface deflections, serves an essential tool for its structural evaluation. However its verification has not been fully conducted. Two methods are available, one is static back calculation which utilizes a set of peak surface deflections and the other dynamic back calculation which employs a set of time series deflection data. FWD tests were conducted at pavement test sites and pavement layer moduli are back calculated by both methods. The result of structural evaluation is verified by comparing the measured strains with computed by utilizing these layer moduli and FWD loadings.

DEVELOPMENT OF DYNAMIC BACKCALCULATION SOFTWARE“Easy DBALM for Windows” FOR FWD TESTS

Data obtained from a FWD test are time series of loading and surface deflections at several locations, since it is an impulsive test. Thus dynamic backcalculation is more suitable than static one because the former conforms to a nature of the test. The software based on the concept is called DBALM (Dynamic Backcalculation for Layer Moduli). DBALM, which was originally based on MS-DOS command line interface, has been revised and simplified by implementing GUI. The windows version of DBALM is called “Easy DBALM for Windows”. This development has brought simplification of input data and visualization of output.